Conventionally, substrate glass for various kinds of displays, particularly, glass on which surface a metal or oxide thin film is formed needs to be alkali-free glass which does not substantially contain alkali metal ions since film characteristics are degraded due to alkali metal ions being diffused in the thin film if an alkali metal oxide is contained.
When a flat panel display (FPD) represented by a liquid crystal display device (LCD) is produced, two sheets of substrate glass (in a case of an LCD, substrate glass provided with a TFT element and substrate glass provided with a color filter) constituting the FPD are bonded to each other by using a curable resin.
Here, since the FPD contains constituent elements such as a TFT element, which has a problem of heat resistance, a photo-curable resin is used as the curable resin and the resin is cured by applying UV rays. Accordingly, it is necessary for substrate glass for display to have UV-transmitting properties. Patent Documents 1 and 2 suggest alkali-free glass substrates whose UV transmittance at a wavelength of 300 nm is in the range of 50% to 85% in terms of 0.5 mm thickness.
The substrate glass for display preferably has a uniform UV transmittance over the entire surface of the substrate glass for display, that is, has a small in-plane distribution of UV transmittance, in addition to the UV transmittance falling within a desired range. For example, there has been a technique of polymer stabilized alignment (hereinafter, referred to as “PSA”) in which a photopolymerizable monomer is added to liquid crystal materials filling a liquid crystal layer of an LCD and is made to form a polymer in the vicinity of an alignment film by applying light in a state in which liquid crystal molecules are tilted in a predetermined direction, thereby stabilizing the molecular alignment of the liquid crystal materials (see Patent Document 3). In order to deal with this technique, it is necessary for substrate glass for display to have a small in-plane distribution of UV transmittance. In the substrate glass for display, when the in-plane distribution of UV transmittance is large, the application conditions of UV rays need to be changed depending on the region of the substrate glass for display and thus the yield of a liquid crystal display panel is decreased.
Further, in the field of an LCD or an organic EL display device (OLED), particularly, mobile display devices such as a mobile and a mobile phone, an important issue is to reduce a weight and thickness of display devices. In order to deal with this issue, it is desirable to make the plate thickness of the substrate glass for display used for a display device smaller. As a method for making the plate thickness smaller, usually applied is a method of performing an etching treatment on substrate glass by using hydrofluoric acid or the like after or before a member for a display device is formed on the surface of the substrate glass, and further thinning by a physically polishing, if necessary.
However, when the substrate glass is made thinner by performing an etching treatment or the like before a member for a display device is formed on the surface of the substrate glass, the strength of the substrate glass is degraded and the deflection amount thereof becomes larger. This causes a problem of incapability of performing a treatment in the existing production line.
In addition, when the substrate glass is made thinner by performing an etching treatment or the like after a member for a display device is formed on the surface of the substrate glass, a problem that small scratches formed on the surface of the substrate glass during the process of forming the member for a display device on the surface of the substrate glass become apparent, that is, a problem of generation of an edge pit arises.
Then, for the purpose of solving such problems, there has been proposed a method of bonding substrate glass having a small plate thickness (thin substrate glass) to another supporting substrate glass to form a laminate, performing a predetermined treatment for producing a display device in this state, and then separating the thin substrate glass from the supporting substrate glass (see Patent Document 4).
As the method of separating the thin substrate glass from the supporting substrate glass, as in the method described in Patent Document 5, a method of scan-irradiating the surface of a substrate with laser beams can be used.
In order to deal with this technique, it is necessary for the substrate glass for display to have a small in-plane distribution of light transmittance. In the substrate glass for display, when the in-plane distribution of light transmittance is large, the application conditions of laser beams need to be changed depending on the region of the substrate glass for display and thus the yield of a display device is decreased.